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Paleobotanical Section

Tomescu, Alexandru Mihail Florian [1], Rothwell, Gar W. [2].

Bacterial microfossils within a macrophytic cyanobacterial colony from the Passage Creek biota (Early Silurian, Virginia, USA).

Compression macrofossils of the Early Silurian Passage Creek biota (Llandoverian, Virginia, USA) reveal profuse colonization by a diverse array of microorganisms. While some of these represent recent contamination by endolithic and soil microbiota, the origin of others is more equivocal. Two of the criteria for establishing the fossil status of microbiota are indigenousness to the fossiliferous rock and syngenicity. Association of the microorganisms with macrofossil material reduces verification of both criteria to documenting whether microorganisms are embedded in the carbonaceous macrofossils, or restricted to surfaces open in fissures. Very small sizes render the position of most of these microorganisms with respect to the macrofossils inconclusive, making it difficult to assess indigenousness/syngenicity. A carbonaceous compression representing a macrophytic cyanobacterial colony harbors features strongly reminiscent of bacteria. These occur as inclusions in the amorphous extracellular matrix of the colony. SEM reveals sub-micron rod-shaped bacteriomorphs and suggests these are embedded in the macrofossil material. Indigenousness and syngenicity of bacteriomorphs are confirmed by TEM which shows them as strings of short rods completely embedded in the carbonaceous matrix. The rounded edges and arrangement confirm a biogenic origin, and together with size identify them as bacteria. They are preserved as internal casts produced by mineral replacement: infilling of voids created by decay of cell contents with precipitated minerals. The bacteria could represent decomposers or symbionts of the cyanobacterial colony. In diverse extant terrestrial and aquatic cyanobacteria, extracellular investments constitute a matrix supporting microhabitats that are often the sites of intense bacterial colonization, and bacterial-cyanobacterial interactions are most commonly mutualistic. Irrespective of the type of relationships between bacteria and cyanobacteria at Passage Creek, this is the earliest direct evidence for bacterial-cyanobacterial interactions and reinforces the importance of this fossil biota for understanding the early stages of continental colonization.


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1 - Humboldt State University, Department of Biological Sciences, Arcata, California, 95521, USA
2 - Ohio University, Department of Environmental & Plant Biology, Porter Hall, Richland Avenue, Athens, Ohio, 45701-2979, USA

Keywords:
bacteria
cyanobacteria
fossil
Silurian
Virginia
mineral .

Presentation Type: Oral Paper
Session: 10-1
Location: 404/Hilton
Date: Monday, August 15th, 2005
Time: 10:15 AM
Abstract ID:149


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